A circulation channel, or a ring channel, is usually an upwardly open endless basin that is used during biological treatment or oxidation of liquid, especially waste water. The waste water/liquid is made to flow along the circulation channel and is thereby made to pass different zones in the circulation channel.
During such biological treatment the waste water is usually purified from nitrogen and biological material by having micro organisms breaking down the biological material into carbon dioxide and water, and by having bacteria transforming the water bound nitrogen to aerial nitrogen. Purified waste water is released into the nature and in the case the water bound nitrogen is not eliminated there is a risk for eutrophication in the natural watercourses, and due to the fact that the biological material is consuming considerable amounts of oxygen watercourses deficient in oxygen are generated if insufficiently purified water is released. The breaking down of the biological material is stimulated by adding large amounts of oxygen to the waste water by means of one or more aeration sectors, and the elimination of the water bound nitrogen takes place in the circulation channel in areas without added oxygen or in separate basins without added oxygen and/or in areas/basins in which the dissolved oxygen level is low enough.
In one, or a few, locations along the circulation channel oxygen is supplied to the waste water by means of mechanical surface aerators, bottom located aerator sections, jet aerators, etc. The micro organisms in the so-called activated sludge consume the oxygen in order to break down the biological material present in the waste water, as well as possibly for nitrification of inter alia ammonium nitrate.
Flow generating machines/mixer machines are used in circulation channels in order to mix the liquid/waste water in order to obtain an as homogenous liquid mixture as possible, keeping the biological material suspended, as well as generating a liquid flow circulating/flowing along the circulation channel.
In some known processes of liquid treatment it is requested that the liquid flow speed along the circulation channel is kept at a predetermined constant level. In a theoretical circulation channel having only one flow generating machine and homogenous liquid, this is easily fulfilled by operating the flow generating machine at a constant operational speed. However, in reality the liquid is not homogenous over time and thereto the treatment plant comprises equipment that are arranged in the circulation channel and that effect the momentum of the flowing liquid in positive direction or in negative direction. A change in the momentum of the liquid flow entails that the speed of the liquid flow is effected/changed. The effect of these equipment may also be variable over time due to varying operation. Based on the fact that the status of the incoming waste water is changed over time, the amount of added oxygen must also be modified over time, and modified intensity entails that the speed of the liquid flow is effected.
The changing speed of the liquid flow may be compensated by revising the operational speed of the flow generating machine, however, this demands for the speed of the liquid flow being monitored by means of expensive speed sensors that are susceptible to disturbances and require regular maintenance.